Metal T-gates have been used for some time to improve the microwave performance of III-IV FET's by simultaneously providing low gate resistance and low parasitic capacitance. T-gates are not common in silicon MOS devices since gate resistance does not usually limit performance in digital circuits. Although MOS devices are rarely used in microwave applications, recent advances have created interest in MOS technology as a low cost alternative in the current gallium arsenide and silicon bipolar technologies. In conventional MOS devices, gate resistance is the dominant factor limiting the microwave performance of the device, thus a T-gate providing a low gate resistance and low parasitic capacitance is ideally suited for these applications.
A. K. Agarwal et al, in their article "MICROX--An Advanced Silicon Technology For Microwave Circuits Up To X-Band," published in the 1991 IEDM TECH DIGEST, pp. 687-690, discloses a microwave SOI process with T-gates. The T-gates are fabricated by first forming a conventional gate then using e-beam lithography to align and pattern a gold cap on the gate. However, both the use of e-beam lithography and gold are inconsistent with the goals of low cost technology.
Nasr, in U.S. Pat. No. 4,912,061, discloses an alternative for forming a cobalt silicide self-aligned metal oxide semiconductor device using disposable silicon nitride spacers.